This invention relates to two wire transmitters, and more particularly to two wire transmitters for generating a loop current that is representative of flow rate through an orifice. Basically, two wire transmitters operate in a system that include a loop supply voltage, a pair of wires extending from the loop supply to some location remote from the supply where a measurement is to be made, and the two wire transmitter at the remote location for making the measurement. The loop supply voltage supplies energy to the transmitter; and the transmitter acts as a variable resistance device whose resistance is varied such that the current in the wires is representative of the measurement made.
One common measurement that is made by two wire transmitters in the process control industry is the pressure drop of a fluid in motion through an orifice. Knowing the size of the orifice, and the pressure drop, the flow rate through the orifice may then be calculated. Flow rate is proportional to the square root of the pressure drop. Currently, two wire transmitters are available to measure this pressure drop and transmit a current in the loop proportional to the drop. However, no devices are available which generate a loop current that is directly proportional to the flow rate.
Thus, it is desirable to have a device which can be added to an existing two wire system that measures pressure drop which will modify the loop current such that it is proportional to a corresponding flow rate. Further, it is desirable for the modified loop current to have the same minimum value and the same maximum value as original loop current. This is because, in the two wire transmitter industry, standards exist for the end points of the loop current. One widely followed standard is that the loop current vary between 4 milliamps and 20 milliamps. These standard end points must be made to correspond to a minimum flow rate and a maximum flow rate in order for other equipment in the loop, such as recorders, to properly operate.
In addition, it is necessary for the device to draw only a small amount of power from the loop. This is because in general, the loop supply voltage must be relatively small for safety reasons. Typically, the loop supply is only 24 volts. And this voltage is partially dropped across the pressure transmitter, and partially dropped across other devices such as recorders or alarms which may be in the loop. Thus, the voltage which remains to be dropped across the disclosed device typically is only on the order of 5 to 10 volts. Further, the minimum loop current using the above described industry standard is only 4 milliamps. Therefore, only several milliwatts of power are available to energize the disclosed device.
Therefore, it is one object of the invention to provide apparatus for modifying a loop current in a two wire transmitter system that is representative of pressure drop through an orifice into a loop current that is representative of slow rate through the orifice.
Another object of the invention is to provide apparatus for modifying a loop current representative of the pressure drop to a loop current representative of flow rate wherein both loop currents have identical end points.